“Fast charging” refers herein to charging a battery at a rate of greater than 30 Amperes per 100 Ampere-hours (“Ahrs”) of battery capacity. A goal of fast charging is to bring the state of charge (“SOC”) of a battery from 30% to 80% of full charge in less than about 1.5 hours. Conventional battery chargers typically operate at charging rates that are at or below 20 A per 100 Ahrs of battery capacity, their charging rate decreases early in the charge cycle, and 8-12 hours may be required to restore a battery to full charge.
Previously, with conventional chargers, the time required for recharging has significantly inconvenienced users of high capacity batteries when the battery-powered equipment needs to remain in continuous service. “High capacity batteries” refers herein to batteries of greater than about 100 Ahrs capacity. For example, industrial users for battery-powered material handling equipment such as forklifts, electric carts, and the like have had to trade out batteries for recharging, remove them from vehicles, typically in a central battery room at some distance across the physical plant. Therefore conventional charging results in a number of disadvantages for the industrial user: loss of employee time for non-productive tasks; safety issues due to additional truck travel away from normal work areas within the plant and the need to handle heavy batteries; increased capital expenditures for the two or more batteries required per piece of equipment; and operation of battery-powered equipment at a low SOC, hence higher current, resulting in increased vehicle maintenance. Fast charging has the potential to increase employee productivity by eliminating battery change-outs; increasing safety by eliminating cross-plant trips for battery change-outs and the need for frequent handling of heavy batteries; decreasing capital expenditures by creating a 1 to 1 battery to equipment ratio; and decreasing vehicle maintenance costs because the batteries are operated at higher SOC.
Recreational users of high capacity batteries, such as electric golf carts, have had to contend with low rate battery chargers that require a spent cart to remain at the charge station until the next day. With fast charging, the cart could be made usable in less than 1.5 hours. This capability could also reduce the total number of carts needed in the recreational operation and thereby significantly reduce capital expenditures.
Automotive users of high capacity batteries would also benefit from fast charging of batteries by being able to enjoy fast recharge rates to bring their electric automobile batteries back up. A fast charger in the garage would bring back up the family auto after the commute home to allow a drive to the mall or soccer field in the evening, which may otherwise not be possible in the absence of fast charging. Fast chargers in parking areas could also be used to restore automobile batteries to high SOC levels.
Fast charging could change the way we live by turning the battery, in effect, into an electrochemical “gas tank”. As fast charge battery technology progresses, and batteries with the ability to accept higher rates of charging become available, higher power fast charging becomes even more desirable. Fast charging has the potential to lower battery recharge times dramatically and become an enabling technology for electric motive power applications.
The majority of currently available high rate chargers fall into two categories—ferro-resonant, and silicon controlled rectifier (SCR) phase control. These chargers contain an input section, power conversion section, and an output section. The input section receives an input three-phase supply voltage and conditions this input for coupling to the power conversion section. The power conversion section converts the coupled input to a desired output voltage. Lastly, the output section couples the voltage from the power conversion section with rectifying diodes and filter if applicable.
FIG. 1 (PRIOR ART) shows a typical three-phase AC input ferro-resonant charger circuit 10 of the prior art. This circuit includes input transformers with one primary and two secondary circuits. One secondary circuit is the RC circuit with resonant winding 1 as shown, and the other secondary circuit includes a winding 2 coupled to the battery via rectifying diodes 3. The transformers are arranged to charge only one battery voltage.
The ferro-resonant style charger described above has a number of inherent deficiencies that limit its practical utility. If a user has more than one battery and they are not the same voltage, two different ferro-resonant chargers will be required. Another inherent problem is the different Ampere-hour ratings of various batteries. A manual change is required to tune the ferro-resonant charger to accommodate the various Ampere-hour ratings, thereby increasing the time and complexity of the charging operation. In addition, the selection range is limited for such modifications to accommodate different Ahr battery ratings. The limited control of output voltage that is characteristic of ferro-resonant chargers also prevents maximum charge rates from being achieved throughout the entire charge cycle. Further, ferro-resonant chargers are unable to take significant corrective action to compensate for supply voltage variations. Because the output of the charger is rectified from the ferro-resonant transformers, a large amount of AC voltage is supplied to the battery. This AC component causes extra heating within the battery and presents a thermal management issue that is of particular concern in the application of ferro-resonant chargers to high rate charging. Self-heating occurs in the transformer primary and secondary as well as the rectifying diodes of the ferro-resonant charger. In the ferro-resonant charger circuitry (see FIG. 1), the resonant secondary winding 1 puts all of the energy in the resistor-capacitor, which makes no contribution to delivery of energy to the battery. These various shortcomings cause the ferro-resonant charger to be inefficient in operation, inflexible in voltage output, and non-optimal in its charging profile.
A second major type of prior art charger is the silicon-controlled rectifier (SCR) phase control charger, for which an electrical schematic diagram is shown in FIG. 2 (PRIOR ART). SCR chargers receive three-phase supply voltage, shown entering the AC input conditioning module 12 in FIG. 2, and couple this input to the power conversion section comprising the SCR switch matrix 20. The SCR switch matrix, coupled to phase controller/driver module 14, converts the input to an output waveform that feeds the output section, which preferably includes a large inductive filter 21. Because the SCR matrix is phase-controlled to obtain the desired output, the output inductor must filter a 360 Hertz distorted AC waveform. This circumstance requires the inductor to be large and expensive. If a smaller, less costly inductor is employed, the additional 360 Hertz AC ripple component superimposed on the DC voltage from the charger will cause heating within the battery load and degrade the power factor of the charger. The AC component effectively serves as an added heat source for the internal resistance of the battery.
SCR chargers are able to accommodate multiple voltage outputs by phase control of the SCR switch matrix. The transformer output voltage is selected to effectively charge the highest voltage battery anticipated. As a result, the power factor of such charger is adversely affected when the charger is used to charge lower voltage batteries. FIG. 2A depicts the SCR switch matrix phase control duty cycle, and shows that for lower voltage batteries, the SCR switch matrix phase control duty cycle must be reduced (see waveform 22) and this reduction in turn greatly degrades power factor. The resultant low power factor results in high currents being drawn from the AC input line in proportion to the true power being delivered to the battery.
Additionally, the SCR switch matrix requires that two SCR switches be on at any given time. SCR-based chargers experience both steady state and switching losses, which degrade their overall efficiency. Furthermore, SCR commutation requirements dictate higher current VA ratings for other circuit components, and SCR recovery times significantly slow down the maximum switching frequencies obtainable. Snubber inductors and RC networks may be needed in order to effect current transfer without occurrence of diode recovery problems in such SCR systems. The drive control circuitry for the SCR switch matrix is complex and expensive because there are multiple SCRs to control (e.g., six in the illustrative system shown in FIG. 2). Finally, the input three-phase AC supply must be connected in the correct sequence in order to prevent phase reversal, since phase reversal would result in an SCR switch matrix error.
Full bridge circuits, if considered for fast charging applications, would appear potentially attractive for fabrication of compact chargers, since a bulky transformer would not be required. However, problems would remain in achieving the power levels necessary for very high capacity batteries (>600 Ahrs capacity) required in many industrial applications. The full bridge circuit employs 4 switches, and, when high voltages are switched, the circuit is susceptible to problems relating to the slower switching speed characteristics of high voltage devices and heat generation. Additionally, transients on the line voltage may destroy the switches, since they are not well isolated.
“Buck” regulator circuits are known in battery chargers for standard rate charging applications. The terms “buck regulator” and “buck converter” are used interchangeably herein. However, design of a buck regulator system for fast charging high capacity batteries has not been achieved and faces a number of technical challenges that have heretofore remained unsolved. Available three-phase electric power would have to be transformed, rectified, switched and filtered in a manner accommodating high rate charging. Relative to switching requirements, switches that switch high currents at high frequencies (e.g., greater than about 5000 Hz) are characteristically associated with unacceptable heat generation, and switches that operate at lower frequencies would require unacceptably large and expensive filters. In addition, the currents required for fast charging of high capacity batteries are very high, e.g., hundreds of Amperes, and thus introduce a myriad of problems relating to the electrolytic capacitors needed for the AC input rectifier circuitry. For high capacity battery charging, these capacitors would have to be so large that their pulse current capability would be very low, e.g., on the order of about 30 A each, and they are less effective at high frequencies because they develop higher inductance under such conditions. If such capacitors were of smaller size to provide higher frequency response, they would have even smaller current handling capability, e.g., less than about 5 A each—over one hundred such capacitors would be required to satisfy the pulsed current and frequency response requirements for fast charging of high capacity batteries. The resulting bus structure would be so large and inductive as to render the construction impractical. Additionally, the use of such capacitors, whether of large size or of small size, results in a low power factor.
Faced with these problems, the art has been unable to achieve a truly viable fast charging technology.
Accordingly, the art remains in need of an effective and practical fast charger for high capacity batteries.